Monte Carlo evaluation of a photon pencil kernel algorithm applied to fast neutron therapy treatment planning

Abstract

When dedicated software is lacking, treatment planning for fast neutron therapy is sometimes performed using dose calculation algorithms designed for photon beam therapy. In this work Monte Carlo derived neutron pencil kernels in water were parametrized using the photon dose algorithm implemented in the Nucletron TMS (treatment management system) treatment planning system. A rectangular fast-neutron fluence spectrum with energies 0-40 MeV (resembling a polyethylene filtered p(41)+Be spectrum) was used. Central axis depth doses and lateral dose distributions were calculated and compared with the corresponding dose distributions from Monte Carlo calculations for homogeneous water and heterogeneous slab phantoms. All absorbed doses were normalized to the reference dose at 10 cm depth for a field of radius 5.6 cm in a 30 x 40 x 20 cm3 water test phantom. Agreement to within 7% was found in both the lateral and the depth dose distributions. The deviations could be explained as due to differences in size between the test phantom and that used in deriving the pencil kernel (radius 200 cm, thickness 50 cm). In the heterogeneous phantom, the TMS, with a directly applied neutron pencil kernel, and Monte Carlo calculated absorbed doses agree approximately for muscle but show large deviations for media such as adipose or bone. For the latter media, agreement was substantially improved by correcting the absorbed doses calculated in TMS with the neutron kerma factor ratio and the stopping power ratio between tissue and water. The multipurpose Monte Carlo code FLUKA was used both in calculating the pencil kernel and in direct calculations of absorbed dose in the phantom.

Pencil beam kernel-based dose calculations on CT data for a mixed neutron-gamma fission field applying tissue correction factors

“…In order to check the correction method for different tissue types suggested by Söderberg et al (2003), simulations were run where the phantom dimensions and the tallies on the central axis were kept unchanged but water as transport medium was replaced by soft tissue, bone, and lung tissue or by different sequences of tissue slabs. One slab phantom was modeled to be comparable to the one used by Söderberg et al but with the difference that only three tissue types were used in this work and the separation of soft tissue into adipose and muscle was not done.…”

“…For neutrons a similar approach can be used due to the similar exponential depthdependence of the neutron attenuation. Söderberg et al (2003) suggest that for the case of neutron propagation through tissue the linear attenuation coefficient should be replaced by the total macroscopic cross section Σ t and scaled relative to the cross section of water by a tissue correction factor c t ≔ Σ t /Σ w in order to calculate the radiological depth. The total macroscopic cross section is calculated from the total atomic cross section σ t by multiplication with the atom density N. Average atomic cross sections sF t , for the neutron fluence Φ n (E) present at MEDAPP were calculated from energy-dependent cross section data for water, soft tissue, lung tissue, and bone according to the following equation:…”

“…The similarities between neutrons and photons in the characteristics of depth dose curves (DDC) and the exponential attenuation in water motivate the application of dose calculation algorithms originally developed for photons also for neutron radiation (Söderberg et al 2003 andKalet et al 2013). Two sets of PBKs were generated, one for neutron radiation and one for gamma radiation.…”

“…For the purpose of dose calculation using PBKs, the implementation of the PBK algorithm in the open source research treatment planning software matRad was used (Wieser et al 2017). A correction method for tissue heterogeneities suggested by Söderberg et al (2003) was applied.…”

Paving the Road for Modern Particle Therapy – What Can We Learn from the Experience Gained with Fast Neutron Therapy in Munich?

“…Although most of these studies only recruited small patient numbers, for certain indications such as incompletely excised or unresectable salivary gland tumors, neutron therapy not only achieved superior LC ( 17 ) but also improved overall survival (OS) ( 18 ). To further increase the efficacy of neutron therapy and to reduce unwanted side effects, efforts were made to improve treatment conformality by introducing 3-D treatment planning systems ( 19 – 21 ). The Karmanos Cancer Center FNT facility in Detroit, MI, USA even had a delivery system for intensity modulated radiotherapy (IMRT) commissioned, but it was shut down in 2011.…”